The capacity of normal human cells to actively modulate DNA repair enzymes is under investigation. Initially we seek to determne whether DNA repair enzymes involved in base excision repair and the entire base excision repair pathway are under active cell control. Using WI-38 diploid fibroblasts stimulated to proliferate by the addition of serum, we have shown that the uracil DNA glycosylase is induced as these normal human cells traverse the cell cycle. Furthermore, cell mediated DNA repair caused by exposure to methylmethane sulfonate or to sodium bisulfate is also activated as a function of cell cycle.